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Hyperbranched polymers: new selective solvents for extractive distillation and solvent extraction
Separation and Purification Technology 30 (2003) 179 /197 www.elsevier.com/locate/seppur
Hyperbranched polymers: new selective solvents for extractive distillation and solvent extraction, M. Seiler , D. Ko¨hler, W. Arlt Technical University of Berlin, Institut fuer Verfahrenstechnik, Sekr. TK 7, Fachgebiet Thermodynamik und Thermische Verfahrenstechnik, Strasse des 17. Juni 135, D-10623 Berlin, Germany Received 26 June 2002; accepted 16 August 2002
Abstract Ternary vapor /liquid (VLE), liquid /liquid (LLE) and solid /liquid /liquid (SLLE) equilibria of ethanol /water and tetrahydrofuran (THF) /water solutions containing different kinds of hyperbranched polymers are presented. For the system THF /water /hyperbranched polyester a remarkably distinct solutropic phenomenon is observed. Commercially available hyperbranched polyesters and hyperbranched polyesteramides are found to be capable of breaking the ethanol /water and THF /water azeotrope. The experimental results underline the potential of hyperbranched polymers in the field of process engineering, especially as an entrainer for extractive distillation and as selective solvents for solvent extraction. The non-volatility of hyperbranched polymers in combination with their remarkable separation efficiency and selectivity enables new processes for the separation of azeotropic mixtures. The separation approaches proposed might offer a potential for cost-savings in comparison with conventional separation processes. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hyperbranched polymers; Extractive distillation; Solvent extraction; Phase equilibria; Azeotropic mixtures
1. Introduction Hyperbranched polymers are highly branched, polydisperse, three-dimensional macromolecules which, due to their unique structures and properties, have attracted increasing attention [1]. Unlike dendrimers, the randomly branched hyperbranched polymers can be synthesised via one-
step reactions and therefore, represent economically promising products for large-scale industrial applications1. Most of their applications are based on the nature and the large number of functional groups within a molecule. These functionalities allow for the tailoring of their physical and chemical properties and thus provide a powerful
1
PII of original article: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 1 6 3 - 6 PII of Introductory article. S1383-5866(02)00798-3 Corresponding author. Tel.: /49-30-314-22646; fax: /4930-314-22406 E-mail address: [email protected] (M. Seiler).
Companies such as the Perstorp Group (Perstorp, Sweden), DSM Fine Chemicals (Geleen, The Netherlands) and Hyperpolymers (Freiburg, Germany) are already producing hyperbranched polymers on a large scale. Currently, Perstorp synthesises hyperbranched polymers, known as Boltorn products, on a ton-scale for 12 a/kg.
1383-5866/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 1 9 7 - 1
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Nomenclature Latin letters K M P S T w x y Greek letters a b g Subscripts 0 i Superscripts V L Abbreviations VLE LLE SLE SLLE DB THF PG ED wt.%
distribution coefficient (/) molecular weight (kg/kmol) pressure (Pa) selectivity (/) temperature (K) weight fraction (/) liquid phase mol fraction (/) vapor phase mol fraction (/) separation factor (/) separation efficiency (/) activity coefficient (/) pure substance component i vapor phase liquid phase vapor/liquid equilibrium liquid/liquid equilibrium solid/liquid equilibrium solid/liquid /liquid equilibrium degree of branching tetrahydrofuran hyperbranched polyglycerol 1,2-ethanediol mass percent, weight percent
tool to design hyperbranched polymers for a wide variety of applications [1]. Since structural perfection is not a strict prerequisite for most biomedical applications [2], even the area of life science, which*/concerning dendritic macromolecules */ until recently seemed to be reserved for the perfectly structured dendrimers, appears to be a promising field for new applications of hyperbranched polymers [2 /5]. An area of application that, until now, has remained unconsidered in scientific discussions is the field of process engineering. Since the polarity of hyperbranched macromolecules can be adjusted by controlled functionalisation of the end groups, selective solvents (consisting of either pure hyper-
branched polymers or fractions of hyperbranched additives) can be tailored [1]. The remarkable selectivities and capacities of hyperbranched polymers in combination with their low melt viscosity, high solubility and thermal stability can be utilised for the optimisation of a number of separation processes. Only recently, the authors suggested the use of hyperbranched polymers as entrainers for extractive distillation and as selective solvents for liquid /liquid extraction [4,6 /9]. Furthermore, the potential for using hyperbranched polymers for the purification of chemical wastewater by means of hyperbranched self-stabilising emulsion liquid membranes, as selectivity- and capacity-increasing solvent additives for absorption processes as well
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as stationary phases for preparative chromatography was demonstrated [4]. This paper focuses on the separation of azeotropic mixtures using commercially available hyperbranched polymers. Ternary vapor/liquid and liquid/liquid equilibria of azeotropic systems containing hyperbranched polymers are presented. Based on the underlying thermodynamic phenomena the separation of ethanol /water by means of extractive distillation2 as well as the separation of tetrahydrofuran (THF)/water by means of solvent extraction3 are discussed and compared with conventional industrial separation processes.
2. Experimental 2.1. VLE measurements by means of headspace gas chromatography ‘Headspace gas chromatography’ (HSGC) [10] represents an experimental approach, which combines a headspace sampler and a gas chromatograph in order to determine the composition of a vapor phase. If the vapor phase is in equilibrium with a liquid or solid phase, vapor /liquid equilibria (VLE) or solid/vapor equilibria (SVE) can be measured and thermodynamic information such as partial pressures, activity coefficients as well as interaction parameters can be obtained [11 /13]. In this work, HSGC was used for VLE measurements of ternary ethanol/water mixtures containing different kinds of hyperbranched polymers. The chosen experimental approach is accurately described in [14]. 2.2. LLE, SLLE and g i measurements The phase behaviour of the ternary system hyperbranched polyester /THF /water was analysed by phase separation experiments as well as 2 Presented by M. Seiler at the international conference on Distillation and Absorption in Baden-Baden, Germany, 2002. 3 Presented by M. Seiler at the Fachausschußsitzung fu¨r Thermische Zerlegung, Adsorption und Extraktion in Bingen/ Rhein, Germany, 2002.
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Table 1 Hyperbranched polymers used in this work Sample
Molecular weight (g/mol)
Mw/Mn
Provider
Boltorn H20 Boltorn H3200 Hybrane S1200 Hybrane H1500
Mw /2100 Mw /10500 Mn /1200 Mn /1500
1.3 1.6 5 to 10 5 to 10
Perstorp Perstorp DSM DSM
, For further details see also [16,22].
by visual cloud-point measurements according to the experimental procedure described in [14]. Activity coefficients at infinite dilution, g i , were determined by means of the gas /liquid chromatography as described in [15].
2.3. Materials
2.3.1. Hyperbranched polymers Table 1 lists the hyperbranched polymers used. Perstorp Speciality Chemicals AB, Sweden, provided aliphatic hyperbranched polyesters, known as the Boltorn family. The Boltorn samples used (Boltorn H20 and Boltorn H3200) are hydroxyl functional hyperbranched polyesters (see Fig. 1a), which are produced from polyalcohol cores and hydroxy acids. The hyperbranched structures are formed by polymerisation of the particular core with 2,2-dimethylol proponic acid (Bis-MPA) [16]. Boltorn H3200 represents a hyperbranched aliphatic polymer (Boltorn H30), whose hydroxyl functionalities are esterified with saturated fatty acids. DSM, Netherlands, provided hyperbranched polymers, known as the Hybrane family. Hybrane S1200 and Hybrane H1500 are hyperbranched polyesteramides4. The structures are illustrated in Fig. 1b and c, respectively. 4
The chemical name of Hybrane S1200 is 2,5-furandione, dihydro-, polymer with 1,1?-iminobis[2-propanol] and of Hybrane H1500 1,3-isobenzofurandione, hexahydro-, polymer with 1,1?-imino-bis[2-propanol].
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show a remarkable solubility in polar solvents such as water or ethanol. As recently described [14], the extent of inter- and intramolecular hydrogen bond formations for these polymer solutions is the dominating factor for a solvent activity and therefore, determines partition coefficients and separation factors. Since the synthesis of hyperbranched polymers allows for the tailoring of properties such as solubility, solution viscosity, selectivity and capacity, the authors analysed the potential of using hyperbranched polymers as an entrainer for the separation of azeotropic mixtures.
Fig. 1. Structural details of the examined polymers; (a) hyperbranched polyester Boltorn H20, (b) hyperbranched polyesteramide Hybrane H1500, (c) hyperbranched polyesteramide Hybrane S1200.
2.3.2. Solvents Ethanol and tetrahydrofuran (THF) were provided by Merck (Germany) with a purity greater than 99.8 mol%. The solvents were used as delivered. Distilled water was degassed and repeatedly filtered using a 0.02 mm Millipore filter in order to remove dust.
3. Results and discussion 3.1. Hyperbranched polymers as selective solvents for extractive distillation Due to the large number of functional groups, hydroxyl functional hyperbranched polymers
3.1.1. Vapor /liquid equilibria of hyperbranched polymer /ethanol/water solutions Figs. 2 and 3 as well as Table 2 show experimental VLE results for ternary ethanol /water mixtures containing different kinds of hyperbranched polymers: hyperbranched polyesteramides (Hybrane H1500 and Hybrane S1500) and a hyperbranched aliphatic polyester (Boltorn H20). The influence of hyperbranched polymers on VLE and separation factor of the ethanol / water system is compared with the influence of the conventional entrainer 1,2-ethanediol (Figs. 3 and 4). In Figs. 2 and 3, the binary ethanol/water VLE is given by NRTL calculations (see solid line) using parameters from the DETHERM database [17]. As shown in [14] the NRTL calculations are in good agreement with recommended experimental data for the binary ethanol /water VLE. All x , y-diagrams are presented on a pseudo-binary basis, i.e. the weight fraction of the binary ethanol /water solution amounts to wethanolwater /1/wentrainer for hyperbranched polymers or 1,2-ethanediol as an entrainer and is split up according to the binary mol fractions plotted (xethanol/xwater /1). For small concentrations of the used hyperbranched polymers (wpolymer 5/20 wt.%), the interactions between polymer and water and between polymer and ethanol, are approximately of the same intensity. Due to good polymer solubility, intermolecular solvent/polymer interactions seem to dominate over polymer/polymer and intramolecular polymer interactions. However, for wpolymer 5/20 wt.%, the hyperbranched polyester-
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Fig. 2. Experimental VLE results for the systems hyperbranched polyester /ethanol /water and hyperbranched polyesteramides / ethanol /water for different polymer concentrations at Tequilibrium /363.15 K; hyperbranched polyester: Boltorn H20, hyperbranched polyesteramides: Hybrane S1200 and Hybrane H1500.
amides Hybrane H1500 and Hybrane S1200, as well as the hydroxyl functional hyperbranched polyester Boltorn H20, do not have a strong impact on the solvent activities and therefore, they do not affect the vapor/liquid equilibrium and the separation factor of the ethanol /water system at 363.15 K. For polymer concentrations above 20 wt.% and xethanol /0.2, the extent of intermolecular interactions between the individual hyperbranched polymer and water increases, leading to a larger molar vapor fraction of ethanol in comparison with the binary ethanol/water VLE (see Fig. 2 and Table 2). As far as the hyperbranched polyesteramide Hybrane S1200 is concerned, a polymer concen-
tration larger than wpolymer /40 wt.% results in breaking the azeotrope of the ethanol /water system. At xethanol /0.89 (the azeotropic concentration of the binary ethanol/water system at 363.15 K), the ethanol concentration in the vapor phase yethanol and hence the separation factor aethanol,water increases with increasing concentration of Hybrane S1200 until the maximum amount of polymer is dissolved. The ethanol-rich systems, containing a comparable amount of Hybrane H1500 or Boltorn H20, show a molar vapor fraction of ethanol considerably smaller than that of Hybrane S1200 (see Figs. 2 and 3). Even though Hybrane H1500 and Boltorn H20 break the azeotrope at high polymer concentrations
184
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Fig. 3. Influence of hyperbranched polyesteramides (Hybrane), hyperbranched polyester (Boltorn), hyperbranched polyglycerol and the conventional entrainer 1,2-ethanediol on the ethanol /water VLE at Tequilibrium /363.15 K.
(wBoltornH20 ]/0.5; wHybraneH1500 ]/0.7), it can be concluded from Fig. 2 that, due to a comparatively low polymer polarity, Hybrane H1500 and Boltorn H20 do not show such a clear interaction preference for water as the hyperbranched polyesteramide Hybrane S1200. This explanation corresponds to polymer/water interactions, which*/ in contrast to Hybrane S1200*/are much weaker for the hyperbranched polymers Hybrane H1500 and Boltorn H20. Both the solubility behaviour of the individual polymer and the number of functional groups (particularly the functional groups on the surface) of a macromolecule seem to determine the separation factor of the respective ternary system.
The ease of ethanol/water separation by means of distillation as well as the efficiency of an entrainer can be evaluated with the help of the separation factor and the separation efficiency. They are defined as follows: separation factor aethanol;water
Kethanol Kwater
separation efficiency
(yethanol =xethanol )
(1)
(ywater =xwater )
b
entrainer a with ethanol; water entrainer a without ethanol; water
(2)
In Fig. 4 the separation factor aethanol,water and the separation efficiency b are depicted as depen-
I
II
III
II a xethanol
IIb
IIc
IV
III a
III b
V
IV a
IV b
IV c
IV d
VI
Va
Vb
Vc
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
0.1001 0.2995 0.5989 0.8482 0.9507
0.4485 0.5949 0.7337 0.8845 0.9588
0.1005 0.3000 0.6005 0.8504 0.9498
0.4314 0.6135 0.7659 0.9107 0.9672
0.0996 0.3010 0.5959 0.8512 0.9428
0.3979 0.6450 0.8273 0.9420 0.9771
0.1999 0.2998 0.4160 0.5987 0.8018 0.9021
0.5225 0.5708 0.6179 0.7054 0.8274 0.9046
0.2001 0.3999 0.5996 0.8002 0.9010 0.9509
0.5043 0.6367 0.7306 0.8565 0.9204 0.9541
0.2008 0.3993 0.6002 0.8013 0.8503 0.9000 0.9500
0.5191 0.6269 0.7261 0.8412 0.8776 0.9101 0.9507
0.1994 0.4028 0.6005 0.8001 0.8500 0.8999 0.9292 0.9513 0.9724
0.4782 0.6289 0.7428 0.8593 0.8874 0.9192 0.9575 0.9706 0.9832
0.2028 0.4033 0.6050 0.7020 0.8003 0.8498 0.9007 0.9482 0.9701
0.4824 0.6391 0.7629 0.8272 0.8917 0.9133 0.9447 0.9747 0.9860
0.1492 0.3949 0.5489 0.6995 0.8332 0.9009 0.9511 0.9731
0.4038 0.6392 0.7504 0.8440 0.9212 0.9560 0.9807 0.9913
0.1997 0.4000 0.5999 0.7998 0.8499 0.9000 0.9499
0.5026 0.6120 0.7095 0.8323 0.8689 0.9082 0.9487
0.1998 0.3999 0.6000 0.8001 0.8500 0.8999 0.9501
0.4621 0.5883 0.7045 0.8390 0.8732 0.9120 0.9498
0.1992 0.4004 0.6032 0.7999 0.8496 0.8999 0.9498
0.4303 0.5874 0.7142 0.8519 0.8861 0.9196 0.9550
0.4001 0.6989 0.8002 0.8511 0.9018 0.9495
0.6480 0.7936 0.8622 0.8967 0.9285 0.9587
T /363.15 K 0.0230 0.2458 0.1000 0.4394 0.1998 0.5195 0.3002 0.5665 0.4002 0.6078 0.5010 0.6521 0.6005 0.7005 0.7035 0.7564 0.8004 0.8210 0.9011 0.9042 0.9476 0.9473 0.9763 0.9753
I, ethanol /water; II, ethanol /water /ethanediol; II a, 20 wt.% ED; II b, 40 wt.% ED; II c, 70 wt.% ED; III, ethanol /water /Boltorn H20; III a, 20 wt.% Boltorn H20; III b, 50 wt.% Boltorn H20; IV, ethanol /water /Hybrane S1200; IV a, 40 wt.% Hybrane S1200; IV b, 60 wt.% Hybrane S1200; IV c, 70 wt.% Hybrane S1200; IV d, 80 wt.% Hybrane S1200; V, ethanol /water /Hybrane H1500; V a, 40 wt.% Hybrane H1500; V b, 60 wt.% Hybrane H1500; V c, 70 wt.% Hybrane H1500; VI, ethanol /water / PEG, 70 wt.% PEG; ED: 1,2-ethanediol, PEG: Poly(ethylene glycol).
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
Table 2 Summary of the experimental VLE headspace results for the systems listed below
185
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Fig. 4. Separation factors and separation efficiencies for ethanol /water mixtures containing hyperbranched polyesteramides (Hybrane S1200 and Hybrane H1500), hyperbranched polyester (Boltorn H20), hyperbranched polyglycerol and 1,2-ethanediol; Tequilibrium / 363.15 K unless otherwise indicated in the legend; note that the line segments between the experimental points represent only a guide for the eyes and no calculated results.
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dent on the pseudo-binary liquid ethanol mol fraction5. Among the studied polymers, the hyperbranched polyesteramide Hybrane S1200 is the most promising candidate as an entrainer in the field of extractive distillation. As can be seen from Fig. 4, for xethanol B/0.7 and polymer concentrations of 70 wt.%, the recently introduced hyperbranched polyglycerol [14,7] and the conventional entrainer 1,2-ethanediol show the best separation efficiency, whereas for the more non-polar Hybrane and Boltorn products the relatively smaller extent of attractive intermolecular water /polymer interactions results in separation efficiencies comparable with those of 40 wt.% ethanediol. Nevertheless, in the important azeotropic ethanol /water concentration range, the separation efficiency curve of Hybrane S1200 shows a large slope for polymer concentrations ]/70 wt.%, resulting in a remarkable separation efficiency of b (xethanol / 0.9, T /363.15 K) /2.4 for wHybraneS1200 /0.8. In comparison to ethanediol or hyperbranched polyglycerol, the maximum separation efficiency of Hybrane S1200 is observed at larger ethanol concentrations (xethanol :/0.95), at which the separation efficiency of Hybrane S1200 reaches the magnitude of the conventional entrainer ethanediol. From Fig. 4 it can be concluded, that the influence of the conventional entrainer 1,2-ethanediol and the hyperbranched polyesteramide Hybrane S1200 on the separation factor aethanol,water is almost of the same magnitude, whereas the hyperbranched aliphatic polyester Boltorn H20 as well as the suggested polymeric entrainer poly(ethylene glycol)6 (see Table 2) 5 For the b-calculations the separation factor of the binary ethanol /water system at 363.15 K was determined using the NRTL-model with parameters from the DETHERM database [17]. 6 Based on only a few VLE data, Al-Amer suggested poly(ethylene glycol) as promising polymeric entrainer for the ethanol /water separation by extractive distillation [18]. Own experimental VLE results for the system ethanol /water / poly(ethylene glycol) can be seen in Table 2. Poly(ethylene glycol) was obtained from Polysciences (Warrington, USA) with a molecular weight of Mw /400 g/mol.
187
exhibit a rather modest effect on the separation factor with only very limited use in the field of extractive distillation. Advantages of using the hyperbranched polyesteramide Hybrane S1200 or the hyperbranched polyglycerol instead of 1,2-ethanediol as entrainer might become evident when focusing on possible, competing operation steps for the entrainer regeneration. In the case of a non-volatile macromolecular entrainer, the problem of an entrainerpolluted overhead product of the extractive distillation column does not exist and a variety of separation steps can be used for the entrainer regeneration (i.e. the separation of the columns’ bottom product) as described in the following section. 3.1.2. Regeneration of hyperbranched macromolecular entrainers The entrainer recovery in a conventional extractive distillation process is mostly carried out using another countercurrent distillation column. Unlike this conventional process, the regeneration of non-volatile entrainers such as hyperbranched polymers allows for the use of other unit operations. As described below, hyperbranched polymers can be separated from low boiling substances like water by means of a stripping column, appropriate thin-film evaporators, dryers or even crystallisers when applicable. Conventional distillation for the separation of a binary mixture consisting of a non-volatile entrainer and a volatile component is not feasible, since the non-volatility of a component would lead to a break down of the columns’ counterflow. A possibility to circumvent this problem is the operation of a stripping column (charge reflux fractionator) without a rectifying section and reflux. A heated inert gas can be fed into the bottoms of the stripper and guided through the column in countercurrent to the entrainer rich feed, resulting in a concentrated entrainer-bottom product and a solvent (water) rich overhead product. Thin-film evaporators represent another alternative for an effective, thermally gentle and continuous recycling of a non-volatile entrainer.
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Fig. 5. Experimental LLE results for the system acetylated hyperbranched polyglycerol /THF /water at Tequilibrium /295.15 K [14]; , cloud points; I, concentration according to LLE measurements; hyperbranched acetylated polyglycerol: Mn /6500 g/mol, Mw/Mn / 2.1, degree of branching/0.56.
Depending on the viscosity of the solution, fallingfilm evaporators or*/for higher viscosities */rotary thin-film evaporators appear to be suitable. Although the hyperbranched polymer melts show comparatively (to other polymers) low viscosities, it might be advantageous to install a refuse worm at the rotory end of the thin-film evaporator such as those used for the stripping of epoxy resins or the degassing of polyolefins. Hyperbranched polymers can also be recycled by convection drying, using a spray dryer, a thinfilm evaporation dryer or a belt dryer. Crystallisation/precipitation, which is dependent on the underlying solid/liquid equilibrium of the polymer-solvent system, could also represent an interesting regeneration alternative. The recovery options described for a nonvolatile entrainer are competing unit operations, which have to be thoroughly assessed with regard to their investment and operation costs. A comparison with conventional separation processes for azeotropic mixtures shows, that the remarkable separation efficiencies of hyperbranched polymers
as well as the variety of energetically promising recycling options for the entrainer offer considerable potential for process and economic optimisations. Therefore, it can be concluded that hyperbranched polymers such as the polyesteramide Hybrane S1200 or the hyperbranched polyglycerol described in [14] represent promising entrainers or entrainer additives for the ethanol /water separation by means of extractive distillation. 3.2. Hyperbranched polymers as selective solvents in liquid /liquid extraction Solvent extraction is a separation process, which is in competition with distillation. However, due to its low energy consumption, solvent extraction is gaining notice. Solvent extraction is often applied if the mixture components exhibit low or high boiling points and thus distillation must be carried out under a costly low temperature or a vacuum operation. It is also applied if the mixture is a close boiling or an azeotropic system, if the components
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
189
Fig. 6. Experimental LLE, SLLE and SLE results for the ternary system hyperbranched polyester /tetrahydrofuran /water at Tequilibrium /321.15 K. LLE measurements, cloud point measurement, SLLE measurements, SLE measurements.
to be separated are thermally sensitive or if the mixture contains a key component of low concentration. Hyperbranched polymers exhibit many properties, which allow for their application as selective solvents in solvent extraction. Large selectivities, remarkable loading capacities for certain key components per mass polymer, low vapor pressures, comparatively low melt and solution viscosities, as well as thermal and chemical stability are only some of their characteristics [14]. A system of industrial importance is the azeotropic mixture of THF and water. THF is produced by dehydrocyclisation of 1,4-butanediol in the presence of an acid catalyst. The key step in purification is the breaking of the THF /water azeotrope [19]. Generally, this is achieved by twopressure (low/high pressure) distillation, extractive distillation or azeotropic distillation. In the following sections, new process approaches for the THF /water separation using hyperbranched polymers as extractive solvents are derived from LLE measurements.
3.2.1. Liquid /liquid equilibria of hyperbranched polymer /THF /water solutions Only recently [14], we discussed the potential of using hyperbranched polymers as selective solvents in the field of solvent extraction by presenting the ternary phase diagram of the system THF /water/ acetylated hyperbranched polyglycerol at 295.15 K (see Fig. 5). THF and water form a minimum boiling azeotrope containing 94.2 wt.% of THF at 0.1 MPa [19]. However, as can be seen from Fig. 5, when extracting THF from water by means of acetylated hyperbranched polyglycerol (Mn /6500 g/mol, Mw/Mn /2.1, degree of branching /0.56) the width of the miscibility gap at 295.15 K as well as the selectivity values are not large enough to break the THF /water azeotrope. Even though the solubility of acetylated hyperbranched polyglycerol in water is sufficiently small, the water solubility in the polymer and hence the water concentration in the polymer rich phase is too large for surpassing the THF /water azeotropic concentration at T/295.15 K [14].
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The extractive separation of THF and water by means of a hyperbranched macromolecular solvent requires a polymer, which */in contrast to the acetylated hyperbranched polyglycerol */shows a higher selectivity and a lower water absorption. Such a polymer represents the commercially available hyperbranched polyester Boltorn H3200. In Figs. 6 and 7 the ternary phase behaviour of the system THF /water /Boltorn H3200 is depicted at 321.15 and 334.15 K in the presence of a vapor phase. At both temperatures, a broad area of immisciblity originates from the binary system Boltorn H3200 /water. At 321.15 K (Fig. 6) and high polymer concentrations, the LLE turns into a solid/liquid /liquid (SLLE) and a solid/liquid (SLE) equilibrium, depending on the polymer concentration. This is due to the melting temperature of Boltorn H3200 (Tmelt,H3200 /333 K), which is larger than the system temperature of 321.15 K. Moreover, as indicated by the slope of the tielines, a solutropic effect of remarkable distinctness was found. Only a few systems such as toluene / water /acetone and n -butyl acetate/water /acetone show solutropic behaviour [20]. In the case
of the ternary system THF /water /Boltorn H3200 the solutropic effect is accompanied by an inversion of the water rich phase and the polymer rich phase. This can be explained by discussing the solubility of THF in both liquid phases and the resulting change of the phase densities in dependence on THF concentration. At low THF concentrations of the LLE region, the polymer rich phase represents the upper liquid phase and the water rich phase represents the lower liquid phase. With increasing THF concentration, for both system temperatures (Figs. 6 and 7), the THF solubility in the polymer rich phase is higher than in the polymer lean phase leading to a negative slope of tie-lines. When approaching the THF concentration in the polymer rich phase of around 50 wt.%, a change in THF distribution can be observed. Any further addition of THF results in an increasing enrichment of THF in the polymer lean phase. Therefore, the negative slope of tielines decreases until the solutropic point, where a horizontal course of the tie-lines can be observed, corresponding to equal THF concentrations in both liquid phases (see Figs. 6 and 7). The location
Fig. 7. Experimental LLE results for the ternary system hyperbranched polyester /tetrahydrofuran /water at Tequilibrium /334.15 K; I, concentration according to LLE measurements; cloud points.
Table 3 Summary of experimental LLE, SLLE, SLE and cloud point results for the systems THF /water /hyperbranched polyester Boltorn H3200 at T/321.15 K and T/334.15 K Polymer lean phase
LLE wTHF
SLLE wpolymer
LLE
SLLE
Cloud points
SLE
wwater
wTHF
wpolymer
wwater
wTHF
wpolymer
wwater
wTHF
wpolymer
wwater
wTHF
wpolymer
wwater
wTHF
wpolymer
wwater
T /321.15 K 0.2290 0.7589 0.3107 0.6757 0.3873 0.5947 0.4611 0.5173 0.5046 0.4722 0.5162 0.4584 0.5399 0.4327 0.5529 0.4212 0.5596 0.4132 0.5776 0.3920 0.6171 0.3468 0.6431 0.3199 0.6597 0.3031
0.0121 0.0136 0.0180 0.0216 0.0232 0.0255 0.0274 0.0259 0.0272 0.0304 0.0361 0.0370 0.0372
0.1274 0.1409 0.1418
0.8652 0.8500 0.8506
0.0074 0.0091 0.0076
0.1434 0.1948 0.2652 0.3218 0.3660 0.4105 0.5035 0.5803 0.6408 0.7240 0.7652 0.7939 0.8483
0.0014 0.0011 0.0031 0.0019 0.0014 0.0019 0.0049 0.0002 0.0073 0.0002 0.0065 0.0108 0.0196
0.8553 0.8041 0.7317 0.6763 0.6325 0.5876 0.4916 0.4195 0.3519 0.2758 0.2283 0.1953 0.1321
0.0747 0.0856 0.0887
0.0005 0.0008 0.0032
0.9248 0.9136 0.9081
0.7479 0.7802 0.8032 0.8200 0.8290 0.8386 0.8509
0.1883 0.1461 0.1179 0.1004 0.0879 0.0746 0.0445
0.0638 0.0737 0.0789 0.0796 0.0831 0.0868 0.1046
0.0373 0.0535 0.0541
0.0005 0.0004 0.0005
0.9622 0.9461 0.9454
T /334.15 K 0.1008 0.8940 0.2665 0.7204 0.3019 0.6850 0.3740 0.6100 0.4740 0.5013 0.5298 0.4382 0.5893 0.3801 0.6237 0.3425 0.6470 0.3195
0.0052 0.0131 0.0131 0.0159 0.0247 0.0320 0.0306 0.0338 0.0335
0.0763 0.1319 0.1546 0.1901 0.2847 0.4022 0.5637 0.7061 0.8088
0.0010 0.0012 0.0013 0.0005 0.0024 0.0010 0.0040 0.0119 0.0514
0.9226 0.8669 0.8441 0.8093 0.7129 0.5968 0.4323 0.2820 0.1398
0.7030 0.7219 0.7618 0.7781 0.7930 0.7992 0.8090 0.8276 0.8305
0.2401 0.2144 0.1646 0.1414 0.1214 0.1121 0.1018 0.0732 0.0664
0.0569 0.0637 0.0736 0.0805 0.0856 0.0887 0.0892 0.0992 0.1031
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
Polymer rich phase
191
192
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Fig. 8. Distribution of THF between the polymer rich and the polymer lean phase for ternary liquid /liquid equilibria of different hyperbranched polymers in THF /water.
of the solutropic point shows a slight temperature dependence. For the equilibrium temperature of 321.15 K (Fig. 6) the solutropic point is located at 54 wt.% THF, whereas for the system temperature of 334.15 K (Fig. 7) the solutropic THF concentration amounts to 60 wt.% (see also Fig. 8). The increased enrichment of THF in the lower polymer lean phase also results in a decreasing density of this phase. Hence, the densities of both liquid phases approach each other leading to similar phase densities in the solutropic area. For THF concentrations above the solutropic point, the THF solubility in the water rich phase is still higher than in the polymer rich phase (positive slope of tie-lines). Therefore, at around 65 wt.% of THF, the density of the water rich phase falls below the density of the polymer rich phase and phase inversion is observed. Apart from these thermodynamic phenomena, it is noteworthy concerning the THF /water separation, that for both system temperatures all phase compositions of the polymer rich phase indicate
the breaking of the atmospheric THF /water azeotrope (see experimental results in Table 3 but on polymer-free basis). For the equilibrium temperatures of 321.15 and 334.15 K, the distribution of THF between the polymer rich and the polymer lean phase is depicted in Fig. 8. Below 50 wt.% of THF, for both polymer solutions, the THF solubility in the polymer rich phase is higher than in the water rich phase. As can be seen for the Boltorn H3200 system, the THF solubility in the polymer rich phase increases with the temperature. At the solutropic point the diagonal is intersected, indicating equal THF concentrations in both liquid phases7. Above around 60 wt.% of THF, due to a higher THF solubility in the polyester lean phase,
7
In the case of the hyperbranched polyglycerol solution, no solutropic effect was measured. Therefore, the intersection with the diagonal represents the critical point of the mixture which can be estimated from Fig. 5.
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193
Fig. 9. Selectivity vs. weight fraction of THF for liquid /liquid equilibria of different hyperbranched polymers in THF /water.
the THF concentration of the water rich phase is larger than for the polymer rich phase. Eventually, the critical point of the Boltorn H3200 mixture, at which the THF concentration in both liquid phases are equal, appears. The preference of Boltorn H3200 for THF as the preferred interaction partner can be recognised from the activity coefficients at infinite dilution. Activity coefficients at infinite dilution of a solute i, g i , provide information about the intermolecular energy between the solvent and the solute and are used, in particular, for the selection of solvents for extraction and extractive distillation [21]. For two different temperatures, T /345.15 K and T /357.17 K, activity coefficients of THF and water infinitely diluted in the hyperbranched polymer Boltorn H3200 were determined [15]. While g water increases from 0.17 (for T /345.15 K) to 0.21 (for T/357.17 K), g THF does not show
a significant temperature dependence (g THF /0.03 for 345.15 K B/T B/357.17 K). The effectiveness of THF extraction by hyperbranched polymer can be characterised by its selectivity S , which is a measure of the ability of the polymer to separate THF from water. For solvent extraction, the selectivity, as defined in Eq. (3), is of comparable importance to the separation factor used in distillation. STHF;water
KTHF w Kwater w
L1 THF L2 THF
wL2 water wL1 water
L1:polymer rich phase; L2:polymer lean phase (3) Based on Table 3, in Fig. 9 the selectivity STHF,water is plotted versus the weight fraction of THF in the polymer rich phase. Unlike hyperbranched acetylated polyglycerol, the Boltorn
194
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Fig. 10. Separation of THF /water by means of a hyperbranched polymer as extraction solvent; A1, A2: countercurrent distillation column, B: mixer /settler, C: polymer /solvent separation unit (e.g. evaporator, dryer); Compositions: THF/polymer/water, concentrations in wt.%.
H3200 system shows remarkable selectivities for the THF /water separation, which gradually decreases with increasing THF concentration in the polymer rich phase. Therefore, it can be concluded that the hyperbranched esterified polyester Boltorn H3200 meets the requirements of an efficient extraction solvent. Thus, new process approaches for the separation of THF /water by means of a hyperbranched polymer can be derived (see next section). 3.2.2. New process approaches for the THF /water separation Based on Fig. 6, the following three separation processes could be suggested for separating a 50 wt.% THF /50 wt.% water mixture: Fig. 10 shows a separation process on the basis of azeotropic distillation. The first distillation column separates the THF /water feed into water as the bottom product and the azeotrope-close THF /water mixture (wTHF :/0.94) as the overhead product. According to Fig. 10, the latter
stream is fed into the second atmospheric countercurrent distillation column, which is operated with a THF-rich reflux. The reflux represents the upper polymer-rich phase in a mixer-settler, whose constant phase compositions could be guaranteed by adjusting the liquid mixture at 321.15 K to that specific tie-line, which leads to an upper liquid phase characterised by wTHF /0.540, wpolymer / 0.433, wwater /0.027 and a lower liquid phase characterised by wTHF /0.503, wpolymer /0.005, wwater /0.492 (see also Fig. 6 and Table 3). Due to the THF-rich reflux, the overall THF concentration in the second distillation column is larger than the azeotropic THF concentration (wTHF /0.942). Therefore, (in spite of the presence of a second liquid phase and a resulting additional mass transfer resistance) a water-free THF /Boltorn H3200 mixture can be received as the bottom product and the minimum boiling azeotrope as the overhead product. The bottom product is separated e.g. by means of a thin-film evaporator into the pure THF product and a highly concentrated
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195
Fig. 11. Separation of THF /water by means of a hyperbranched polymer as extraction solvent, A1, A2: countercurrent distillation column, B: mixer /settler, C: polymer /solvent separation unit (e.g. evaporator, dryer); Compositions: THF/polymer/water, concentrations in wt.%.
polymer solution8, which is conveyed at T / 333K9 into the mixer-settler. In order to save the reboiler of the column A1, a partial flow of the THF vapor product could be fed into the bottom of A1. The liquid mixture, consisting of the regenerated polymer and the condensed overhead product of the second distillation column, should meet the phase composition criteria described above, so that the lower polymer free, liquid phase of the mixer-settler shows the same composition as the initial THF /water feed solution. Therefore, both streams, the lower liquid phase of the mixersettler and the THF /water feed, can be mixed in accordance to the overall mass balance and sub8
Since the mixture in the mixer /settler must have a constant composition, the overall mass balance determines the amount of THF in the highly concentrated polymer solution. Generally, a concentrated polymer solution containing about 10 wt.% of THF appears desirable, since the viscosity of the concentrated polymer stream as well as the energy input of the evaporator have to be taken into account. Future work will focus on these aspects of process optimization (see Section 4). 9 Boltorn H3200 is thermally stable up to 520 K. The melting point of this semi-crystalline polymer is 333 K.
sequently fed into the first distillation column (see Fig. 10). The second separation scheme, depicted in Fig. 11, represents a process with the same unit operations as in Fig. 10. But since the unit operations in Fig. 11 are arranged in a different order and the solvent extraction is based on another tie-line, the second separation scheme (Fig. 11) might be energetically superior to conventional extraction processes and to the separation scheme of Fig. 10. As can be seen in Fig. 11, the first separation step represents the liquid-liquid demixing of the THF /water feed, which is mixed with the regenerated polymer, in a mixer-settler at 321.15 K. In this process another tie-line from Fig. 6 is chosen for the single step extraction, leading to an upper liquid phase characterised by wTHF / 0.461, wpolymer /0.517, wwater /0.022 and a lower liquid phase characterised by wTHF /0.322, wpolymer /0.002, wwater /0.676. The latter waterrich phase is separated by means of a small atmospheric countercurrent distillation column, which */within two or three theoretical stages */ leads to water as the bottom product and a THF /
196
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
water mixture, similar in composition to the feed solution, as the overhead product. The upper polymer-rich phase of the mixer settler with wTHF /0.955 /wTHF,azeotrope /0.942 is fed into a second distillation column. The bottom product of this column, a THF /polymer mixture, is separated (for instance by means of an evaporator or dryer). The yielded polymer is added to the columns’ overhead product (i.e. the low boiling azeotropic THF /water mixture), so that the underlying tieline of the single step extraction is met and the overall mass-balance is satisfied. When necessary, additional polymer is added in order to reach the selected tie-line. The THF /polymer separation is realised in accordance with Fig. 10. A third separation process is conceivable, which*/apart from the separation of the upper liquid phase of the solvent extraction in Fig. 11*/ is equivalent to the latter separation scheme. In this third approach, the upper polymer rich phase from the liquid/liquid extraction is separated by means of (shear) crystallisation into a liquid THF product and a solid phase, containing all water and most of the polymer. The solid mixture is fed into an extruder. Here, devolatilisation allows the water /polymer separation, so that the polymer can be recycled. Further research on the underlying solid-liquid equilibria will investigate the feasibility of crystallisation for the THF /water separation.
4. Conclusions and future work Hyperbranched polymers represent a promising class of highly selective macromolecules, which can be used for the optimisation of a variety of separation processes. Properties such as solubility, capacity, selectivity, melt and solution viscosity, thermal stability as well as glass transition temperature can be tailored according to the individual application. In this work the potential of commercially available hyperbranched polymers in the field of extractive distillation and solvent extraction is demonstrated. In contrast to Hybrane H1500 and Boltorn H20, the hyperbranched polymers Hybrane S1200 and hyperbranched polyglycerol exhibit remarkable
separation efficiencies for the ethanol /water system. Both hyperbranched representatives, Hybrane S1200 and hyperbranched polyglycerol, easily break the azeotropic phase behaviour of the ethanol /water system due to selective interactions with water. For xethanol /0.3, a considerable increase of the separation factor aethanol,water was observed when adding Hybrane S1200 or hyperbranched polyglycerol to the ethanol/water solution. In case of Hybrane S1200 this increase proved to be of the same magnitude as that of the conventional entrainer 1,2-ethanediol, which confirms the suitability of using Hybrane S1200 as an entrainer or entrainer additive for the separation of azeotropic mixtures by extractive distillation. Due to the use of a non-volatile entrainer, the entrainer regeneration can be carried out by stripping, evaporation, drying or crystallisation. Moreover, the main column does not require a section for the separation of the low boiling component from the entrainer, since a polymeric entrainer cannot pollute the columns’ overhead product. Therefore, extractive distillation using one of the suggested hyperbranched polymers as entrainer indicate a potential for cost-savings in comparison to conventional extractive distillation processes. The separation of the azeotropic system THF / water by means of solvent extraction is also studied. The hyperbranched aliphatic polyester Boltorn H3200 is used as a solvent. Boltorn H3200 exhibits a remarkable selectivity and capacity, which allows for the breaking of the THFwater azeotrope by single stage extraction. The purification of the extract and the raffinate can be achieved by combining the extraction unit either with distillation or crystallisation steps. For the system THF/water /hyperbranched polyester, a remarkably distinct solutropic phenomenon is observed. Future work will focus on evaluating the suggested processes for separating azeotropic mixtures by means of hyperbranched polymers. ASPEN PLUS† simulations will be carried out to energetically optimise the individual separation approach and to discuss their advantages and
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
disadvantages in contrast to conventional separation processes. [8]
Acknowledgements The authors would like to thank Perstorp Speciality Chemicals AB for providing hyperbranched aliphatic polyesters. Thanks especially to B. Pettersson and H.C. Bjoernberg, Perstorp, for useful discussions. We also appreciate the support of Dr P.E. Froehling, DSM, who kindly provided hyperbranched polyesteramides. Experimental HSGC-assistance of Dipl.-Ing. A. Kavarnou is gratefully acknowledged.
[9] [10]
[11] [12] [13] [14]
References [15] [1] M. Seiler, Chem. Eng. Technol. 25 (3) (2002) 237. [2] A. Sunder, J. Heinemann, H. Frey, Chem. Eur. J. 6 (2000) 2499. [3] H. Frey, R. Haag, Rev. Mol. Biotechnol. 90 (2002) 257. [4] M. Seiler, W. Arlt, Phase behaviour and new industrial applications of hyperbranched polymers, Polydays Conference, Berlin, Germany, 2002. [5] M. Seiler, I. Smirnova, S. Suttiruengwong, W. Arlt, Int. J. Pharm. (2002) in press. [6] M. Seiler, J. Rolker, W. Arlt, Kritisches Entmischungsverhalten hyperverzweigter Polymerlo¨sungen (Critical demixing behaviour of hyperbranched polymer solutions) GVCFachausschusitzung ‘Hochdruckverfahrenstechnik’, Bochum, VDI-Gesellschaft, Germany, 2002. [7] M. Seiler, D. Ko¨hler, W. Arlt, Hyperbranched polymers */ new classes of selective solvents for liquid /liquid extrac-
[16] [17] [18] [19] [20]
[21] [22]
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tion, Fachausschußsitzungen on ‘Thermische Zerlegung, Adsorption und Extraktion’, Bingen/Rhein, VDI-Gesellschaft, Germany, 2002. M. Seiler, C. Jork, T. Schneider, W. Arlt, Ionic liquids and hyperbranched polymers */promising new classes of selective entrainers for extractive distillation (full paper), in: Proceedings of the International Conference on ‘Distillation and Absorption’, Baden-Baden, Germany, 2002, ISBN 3-931 384-37-3, in press. W. Arlt, M. Seiler, G. Sadowski, H. Frey, H. Kautz, R. Mu¨hlhaupt (2001), DE Patent No. 10160518.8. H. Hachenberg, K. Beringer, Die Headspace-Gaschromatographie als Analysen- und Meßmethode, Vieweg Verlag, Braunschweig/Wiesbaden, 1996. H.M. Petri, B.A. Wolf, Macromolecules 27 (1994) 2714. C.B. Castells, D.I. Eikens, P.W. Carr, J. Chem. Eng. Data 45 (2000) 369. N. Schuld, B.A. Wolf, J. Pol. Sci.: Part B: Pol. Phys. 39 (2001) 651. M. Seiler, W. Arlt, H. Kautz, H. Frey, Fluid Phase Equilib. 201 (2) (2002) 359. G. Sadowski, L.V. Mokrushina, W. Arlt, Fluid Phase Equilib. 139 (1997) 391. W.O. Patent 93/17060, E.P. Patent 0630389, US Patent 5418301 (1995) Perstorp AB (SE). http://i-systems.dechema.de/detherm/ (Accessed March 2002). A.M. Al-Amer, Ind. Eng. Chem. Res. 39 (2000) 3901. T.e. Chang, T.T. Shih, Fluid Phase Equilib. 52 (1989) 161. Standard Test Systems for Liquid Extraction, European Federation of Chemical Engineering, published by the Institute of Chemical Engineers, ISBN 0 85295 181 7, Warwickshire, UK, 1985. A. Heintz, D.V. Kulikov, S.P. Verevkin, J. Chem. Eng. Data 46 (6) (2001) 1526. A. Burgath, A. Sunder, H. Frey, Macromol. Chem. Phys. 201 (2000) 782.
Hyperbranched polymers: new selective solvents for extractive distillation and solvent extraction, M. Seiler , D. Ko¨hler, W. Arlt Technical University of Berlin, Institut fuer Verfahrenstechnik, Sekr. TK 7, Fachgebiet Thermodynamik und Thermische Verfahrenstechnik, Strasse des 17. Juni 135, D-10623 Berlin, Germany Received 26 June 2002; accepted 16 August 2002
Abstract Ternary vapor /liquid (VLE), liquid /liquid (LLE) and solid /liquid /liquid (SLLE) equilibria of ethanol /water and tetrahydrofuran (THF) /water solutions containing different kinds of hyperbranched polymers are presented. For the system THF /water /hyperbranched polyester a remarkably distinct solutropic phenomenon is observed. Commercially available hyperbranched polyesters and hyperbranched polyesteramides are found to be capable of breaking the ethanol /water and THF /water azeotrope. The experimental results underline the potential of hyperbranched polymers in the field of process engineering, especially as an entrainer for extractive distillation and as selective solvents for solvent extraction. The non-volatility of hyperbranched polymers in combination with their remarkable separation efficiency and selectivity enables new processes for the separation of azeotropic mixtures. The separation approaches proposed might offer a potential for cost-savings in comparison with conventional separation processes. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hyperbranched polymers; Extractive distillation; Solvent extraction; Phase equilibria; Azeotropic mixtures
1. Introduction Hyperbranched polymers are highly branched, polydisperse, three-dimensional macromolecules which, due to their unique structures and properties, have attracted increasing attention [1]. Unlike dendrimers, the randomly branched hyperbranched polymers can be synthesised via one-
step reactions and therefore, represent economically promising products for large-scale industrial applications1. Most of their applications are based on the nature and the large number of functional groups within a molecule. These functionalities allow for the tailoring of their physical and chemical properties and thus provide a powerful
1
PII of original article: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 1 6 3 - 6 PII of Introductory article. S1383-5866(02)00798-3 Corresponding author. Tel.: /49-30-314-22646; fax: /4930-314-22406 E-mail address: [email protected] (M. Seiler).
Companies such as the Perstorp Group (Perstorp, Sweden), DSM Fine Chemicals (Geleen, The Netherlands) and Hyperpolymers (Freiburg, Germany) are already producing hyperbranched polymers on a large scale. Currently, Perstorp synthesises hyperbranched polymers, known as Boltorn products, on a ton-scale for 12 a/kg.
1383-5866/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 1 9 7 - 1
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Nomenclature Latin letters K M P S T w x y Greek letters a b g Subscripts 0 i Superscripts V L Abbreviations VLE LLE SLE SLLE DB THF PG ED wt.%
distribution coefficient (/) molecular weight (kg/kmol) pressure (Pa) selectivity (/) temperature (K) weight fraction (/) liquid phase mol fraction (/) vapor phase mol fraction (/) separation factor (/) separation efficiency (/) activity coefficient (/) pure substance component i vapor phase liquid phase vapor/liquid equilibrium liquid/liquid equilibrium solid/liquid equilibrium solid/liquid /liquid equilibrium degree of branching tetrahydrofuran hyperbranched polyglycerol 1,2-ethanediol mass percent, weight percent
tool to design hyperbranched polymers for a wide variety of applications [1]. Since structural perfection is not a strict prerequisite for most biomedical applications [2], even the area of life science, which*/concerning dendritic macromolecules */ until recently seemed to be reserved for the perfectly structured dendrimers, appears to be a promising field for new applications of hyperbranched polymers [2 /5]. An area of application that, until now, has remained unconsidered in scientific discussions is the field of process engineering. Since the polarity of hyperbranched macromolecules can be adjusted by controlled functionalisation of the end groups, selective solvents (consisting of either pure hyper-
branched polymers or fractions of hyperbranched additives) can be tailored [1]. The remarkable selectivities and capacities of hyperbranched polymers in combination with their low melt viscosity, high solubility and thermal stability can be utilised for the optimisation of a number of separation processes. Only recently, the authors suggested the use of hyperbranched polymers as entrainers for extractive distillation and as selective solvents for liquid /liquid extraction [4,6 /9]. Furthermore, the potential for using hyperbranched polymers for the purification of chemical wastewater by means of hyperbranched self-stabilising emulsion liquid membranes, as selectivity- and capacity-increasing solvent additives for absorption processes as well
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as stationary phases for preparative chromatography was demonstrated [4]. This paper focuses on the separation of azeotropic mixtures using commercially available hyperbranched polymers. Ternary vapor/liquid and liquid/liquid equilibria of azeotropic systems containing hyperbranched polymers are presented. Based on the underlying thermodynamic phenomena the separation of ethanol /water by means of extractive distillation2 as well as the separation of tetrahydrofuran (THF)/water by means of solvent extraction3 are discussed and compared with conventional industrial separation processes.
2. Experimental 2.1. VLE measurements by means of headspace gas chromatography ‘Headspace gas chromatography’ (HSGC) [10] represents an experimental approach, which combines a headspace sampler and a gas chromatograph in order to determine the composition of a vapor phase. If the vapor phase is in equilibrium with a liquid or solid phase, vapor /liquid equilibria (VLE) or solid/vapor equilibria (SVE) can be measured and thermodynamic information such as partial pressures, activity coefficients as well as interaction parameters can be obtained [11 /13]. In this work, HSGC was used for VLE measurements of ternary ethanol/water mixtures containing different kinds of hyperbranched polymers. The chosen experimental approach is accurately described in [14]. 2.2. LLE, SLLE and g i measurements The phase behaviour of the ternary system hyperbranched polyester /THF /water was analysed by phase separation experiments as well as 2 Presented by M. Seiler at the international conference on Distillation and Absorption in Baden-Baden, Germany, 2002. 3 Presented by M. Seiler at the Fachausschußsitzung fu¨r Thermische Zerlegung, Adsorption und Extraktion in Bingen/ Rhein, Germany, 2002.
181
Table 1 Hyperbranched polymers used in this work Sample
Molecular weight (g/mol)
Mw/Mn
Provider
Boltorn H20 Boltorn H3200 Hybrane S1200 Hybrane H1500
Mw /2100 Mw /10500 Mn /1200 Mn /1500
1.3 1.6 5 to 10 5 to 10
Perstorp Perstorp DSM DSM
, For further details see also [16,22].
by visual cloud-point measurements according to the experimental procedure described in [14]. Activity coefficients at infinite dilution, g i , were determined by means of the gas /liquid chromatography as described in [15].
2.3. Materials
2.3.1. Hyperbranched polymers Table 1 lists the hyperbranched polymers used. Perstorp Speciality Chemicals AB, Sweden, provided aliphatic hyperbranched polyesters, known as the Boltorn family. The Boltorn samples used (Boltorn H20 and Boltorn H3200) are hydroxyl functional hyperbranched polyesters (see Fig. 1a), which are produced from polyalcohol cores and hydroxy acids. The hyperbranched structures are formed by polymerisation of the particular core with 2,2-dimethylol proponic acid (Bis-MPA) [16]. Boltorn H3200 represents a hyperbranched aliphatic polymer (Boltorn H30), whose hydroxyl functionalities are esterified with saturated fatty acids. DSM, Netherlands, provided hyperbranched polymers, known as the Hybrane family. Hybrane S1200 and Hybrane H1500 are hyperbranched polyesteramides4. The structures are illustrated in Fig. 1b and c, respectively. 4
The chemical name of Hybrane S1200 is 2,5-furandione, dihydro-, polymer with 1,1?-iminobis[2-propanol] and of Hybrane H1500 1,3-isobenzofurandione, hexahydro-, polymer with 1,1?-imino-bis[2-propanol].
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show a remarkable solubility in polar solvents such as water or ethanol. As recently described [14], the extent of inter- and intramolecular hydrogen bond formations for these polymer solutions is the dominating factor for a solvent activity and therefore, determines partition coefficients and separation factors. Since the synthesis of hyperbranched polymers allows for the tailoring of properties such as solubility, solution viscosity, selectivity and capacity, the authors analysed the potential of using hyperbranched polymers as an entrainer for the separation of azeotropic mixtures.
Fig. 1. Structural details of the examined polymers; (a) hyperbranched polyester Boltorn H20, (b) hyperbranched polyesteramide Hybrane H1500, (c) hyperbranched polyesteramide Hybrane S1200.
2.3.2. Solvents Ethanol and tetrahydrofuran (THF) were provided by Merck (Germany) with a purity greater than 99.8 mol%. The solvents were used as delivered. Distilled water was degassed and repeatedly filtered using a 0.02 mm Millipore filter in order to remove dust.
3. Results and discussion 3.1. Hyperbranched polymers as selective solvents for extractive distillation Due to the large number of functional groups, hydroxyl functional hyperbranched polymers
3.1.1. Vapor /liquid equilibria of hyperbranched polymer /ethanol/water solutions Figs. 2 and 3 as well as Table 2 show experimental VLE results for ternary ethanol /water mixtures containing different kinds of hyperbranched polymers: hyperbranched polyesteramides (Hybrane H1500 and Hybrane S1500) and a hyperbranched aliphatic polyester (Boltorn H20). The influence of hyperbranched polymers on VLE and separation factor of the ethanol / water system is compared with the influence of the conventional entrainer 1,2-ethanediol (Figs. 3 and 4). In Figs. 2 and 3, the binary ethanol/water VLE is given by NRTL calculations (see solid line) using parameters from the DETHERM database [17]. As shown in [14] the NRTL calculations are in good agreement with recommended experimental data for the binary ethanol /water VLE. All x , y-diagrams are presented on a pseudo-binary basis, i.e. the weight fraction of the binary ethanol /water solution amounts to wethanolwater /1/wentrainer for hyperbranched polymers or 1,2-ethanediol as an entrainer and is split up according to the binary mol fractions plotted (xethanol/xwater /1). For small concentrations of the used hyperbranched polymers (wpolymer 5/20 wt.%), the interactions between polymer and water and between polymer and ethanol, are approximately of the same intensity. Due to good polymer solubility, intermolecular solvent/polymer interactions seem to dominate over polymer/polymer and intramolecular polymer interactions. However, for wpolymer 5/20 wt.%, the hyperbranched polyester-
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Fig. 2. Experimental VLE results for the systems hyperbranched polyester /ethanol /water and hyperbranched polyesteramides / ethanol /water for different polymer concentrations at Tequilibrium /363.15 K; hyperbranched polyester: Boltorn H20, hyperbranched polyesteramides: Hybrane S1200 and Hybrane H1500.
amides Hybrane H1500 and Hybrane S1200, as well as the hydroxyl functional hyperbranched polyester Boltorn H20, do not have a strong impact on the solvent activities and therefore, they do not affect the vapor/liquid equilibrium and the separation factor of the ethanol /water system at 363.15 K. For polymer concentrations above 20 wt.% and xethanol /0.2, the extent of intermolecular interactions between the individual hyperbranched polymer and water increases, leading to a larger molar vapor fraction of ethanol in comparison with the binary ethanol/water VLE (see Fig. 2 and Table 2). As far as the hyperbranched polyesteramide Hybrane S1200 is concerned, a polymer concen-
tration larger than wpolymer /40 wt.% results in breaking the azeotrope of the ethanol /water system. At xethanol /0.89 (the azeotropic concentration of the binary ethanol/water system at 363.15 K), the ethanol concentration in the vapor phase yethanol and hence the separation factor aethanol,water increases with increasing concentration of Hybrane S1200 until the maximum amount of polymer is dissolved. The ethanol-rich systems, containing a comparable amount of Hybrane H1500 or Boltorn H20, show a molar vapor fraction of ethanol considerably smaller than that of Hybrane S1200 (see Figs. 2 and 3). Even though Hybrane H1500 and Boltorn H20 break the azeotrope at high polymer concentrations
184
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Fig. 3. Influence of hyperbranched polyesteramides (Hybrane), hyperbranched polyester (Boltorn), hyperbranched polyglycerol and the conventional entrainer 1,2-ethanediol on the ethanol /water VLE at Tequilibrium /363.15 K.
(wBoltornH20 ]/0.5; wHybraneH1500 ]/0.7), it can be concluded from Fig. 2 that, due to a comparatively low polymer polarity, Hybrane H1500 and Boltorn H20 do not show such a clear interaction preference for water as the hyperbranched polyesteramide Hybrane S1200. This explanation corresponds to polymer/water interactions, which*/ in contrast to Hybrane S1200*/are much weaker for the hyperbranched polymers Hybrane H1500 and Boltorn H20. Both the solubility behaviour of the individual polymer and the number of functional groups (particularly the functional groups on the surface) of a macromolecule seem to determine the separation factor of the respective ternary system.
The ease of ethanol/water separation by means of distillation as well as the efficiency of an entrainer can be evaluated with the help of the separation factor and the separation efficiency. They are defined as follows: separation factor aethanol;water
Kethanol Kwater
separation efficiency
(yethanol =xethanol )
(1)
(ywater =xwater )
b
entrainer a with ethanol; water entrainer a without ethanol; water
(2)
In Fig. 4 the separation factor aethanol,water and the separation efficiency b are depicted as depen-
I
II
III
II a xethanol
IIb
IIc
IV
III a
III b
V
IV a
IV b
IV c
IV d
VI
Va
Vb
Vc
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
xetha-
yetha-
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
nol
0.1001 0.2995 0.5989 0.8482 0.9507
0.4485 0.5949 0.7337 0.8845 0.9588
0.1005 0.3000 0.6005 0.8504 0.9498
0.4314 0.6135 0.7659 0.9107 0.9672
0.0996 0.3010 0.5959 0.8512 0.9428
0.3979 0.6450 0.8273 0.9420 0.9771
0.1999 0.2998 0.4160 0.5987 0.8018 0.9021
0.5225 0.5708 0.6179 0.7054 0.8274 0.9046
0.2001 0.3999 0.5996 0.8002 0.9010 0.9509
0.5043 0.6367 0.7306 0.8565 0.9204 0.9541
0.2008 0.3993 0.6002 0.8013 0.8503 0.9000 0.9500
0.5191 0.6269 0.7261 0.8412 0.8776 0.9101 0.9507
0.1994 0.4028 0.6005 0.8001 0.8500 0.8999 0.9292 0.9513 0.9724
0.4782 0.6289 0.7428 0.8593 0.8874 0.9192 0.9575 0.9706 0.9832
0.2028 0.4033 0.6050 0.7020 0.8003 0.8498 0.9007 0.9482 0.9701
0.4824 0.6391 0.7629 0.8272 0.8917 0.9133 0.9447 0.9747 0.9860
0.1492 0.3949 0.5489 0.6995 0.8332 0.9009 0.9511 0.9731
0.4038 0.6392 0.7504 0.8440 0.9212 0.9560 0.9807 0.9913
0.1997 0.4000 0.5999 0.7998 0.8499 0.9000 0.9499
0.5026 0.6120 0.7095 0.8323 0.8689 0.9082 0.9487
0.1998 0.3999 0.6000 0.8001 0.8500 0.8999 0.9501
0.4621 0.5883 0.7045 0.8390 0.8732 0.9120 0.9498
0.1992 0.4004 0.6032 0.7999 0.8496 0.8999 0.9498
0.4303 0.5874 0.7142 0.8519 0.8861 0.9196 0.9550
0.4001 0.6989 0.8002 0.8511 0.9018 0.9495
0.6480 0.7936 0.8622 0.8967 0.9285 0.9587
T /363.15 K 0.0230 0.2458 0.1000 0.4394 0.1998 0.5195 0.3002 0.5665 0.4002 0.6078 0.5010 0.6521 0.6005 0.7005 0.7035 0.7564 0.8004 0.8210 0.9011 0.9042 0.9476 0.9473 0.9763 0.9753
I, ethanol /water; II, ethanol /water /ethanediol; II a, 20 wt.% ED; II b, 40 wt.% ED; II c, 70 wt.% ED; III, ethanol /water /Boltorn H20; III a, 20 wt.% Boltorn H20; III b, 50 wt.% Boltorn H20; IV, ethanol /water /Hybrane S1200; IV a, 40 wt.% Hybrane S1200; IV b, 60 wt.% Hybrane S1200; IV c, 70 wt.% Hybrane S1200; IV d, 80 wt.% Hybrane S1200; V, ethanol /water /Hybrane H1500; V a, 40 wt.% Hybrane H1500; V b, 60 wt.% Hybrane H1500; V c, 70 wt.% Hybrane H1500; VI, ethanol /water / PEG, 70 wt.% PEG; ED: 1,2-ethanediol, PEG: Poly(ethylene glycol).
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
Table 2 Summary of the experimental VLE headspace results for the systems listed below
185
186
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Fig. 4. Separation factors and separation efficiencies for ethanol /water mixtures containing hyperbranched polyesteramides (Hybrane S1200 and Hybrane H1500), hyperbranched polyester (Boltorn H20), hyperbranched polyglycerol and 1,2-ethanediol; Tequilibrium / 363.15 K unless otherwise indicated in the legend; note that the line segments between the experimental points represent only a guide for the eyes and no calculated results.
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
dent on the pseudo-binary liquid ethanol mol fraction5. Among the studied polymers, the hyperbranched polyesteramide Hybrane S1200 is the most promising candidate as an entrainer in the field of extractive distillation. As can be seen from Fig. 4, for xethanol B/0.7 and polymer concentrations of 70 wt.%, the recently introduced hyperbranched polyglycerol [14,7] and the conventional entrainer 1,2-ethanediol show the best separation efficiency, whereas for the more non-polar Hybrane and Boltorn products the relatively smaller extent of attractive intermolecular water /polymer interactions results in separation efficiencies comparable with those of 40 wt.% ethanediol. Nevertheless, in the important azeotropic ethanol /water concentration range, the separation efficiency curve of Hybrane S1200 shows a large slope for polymer concentrations ]/70 wt.%, resulting in a remarkable separation efficiency of b (xethanol / 0.9, T /363.15 K) /2.4 for wHybraneS1200 /0.8. In comparison to ethanediol or hyperbranched polyglycerol, the maximum separation efficiency of Hybrane S1200 is observed at larger ethanol concentrations (xethanol :/0.95), at which the separation efficiency of Hybrane S1200 reaches the magnitude of the conventional entrainer ethanediol. From Fig. 4 it can be concluded, that the influence of the conventional entrainer 1,2-ethanediol and the hyperbranched polyesteramide Hybrane S1200 on the separation factor aethanol,water is almost of the same magnitude, whereas the hyperbranched aliphatic polyester Boltorn H20 as well as the suggested polymeric entrainer poly(ethylene glycol)6 (see Table 2) 5 For the b-calculations the separation factor of the binary ethanol /water system at 363.15 K was determined using the NRTL-model with parameters from the DETHERM database [17]. 6 Based on only a few VLE data, Al-Amer suggested poly(ethylene glycol) as promising polymeric entrainer for the ethanol /water separation by extractive distillation [18]. Own experimental VLE results for the system ethanol /water / poly(ethylene glycol) can be seen in Table 2. Poly(ethylene glycol) was obtained from Polysciences (Warrington, USA) with a molecular weight of Mw /400 g/mol.
187
exhibit a rather modest effect on the separation factor with only very limited use in the field of extractive distillation. Advantages of using the hyperbranched polyesteramide Hybrane S1200 or the hyperbranched polyglycerol instead of 1,2-ethanediol as entrainer might become evident when focusing on possible, competing operation steps for the entrainer regeneration. In the case of a non-volatile macromolecular entrainer, the problem of an entrainerpolluted overhead product of the extractive distillation column does not exist and a variety of separation steps can be used for the entrainer regeneration (i.e. the separation of the columns’ bottom product) as described in the following section. 3.1.2. Regeneration of hyperbranched macromolecular entrainers The entrainer recovery in a conventional extractive distillation process is mostly carried out using another countercurrent distillation column. Unlike this conventional process, the regeneration of non-volatile entrainers such as hyperbranched polymers allows for the use of other unit operations. As described below, hyperbranched polymers can be separated from low boiling substances like water by means of a stripping column, appropriate thin-film evaporators, dryers or even crystallisers when applicable. Conventional distillation for the separation of a binary mixture consisting of a non-volatile entrainer and a volatile component is not feasible, since the non-volatility of a component would lead to a break down of the columns’ counterflow. A possibility to circumvent this problem is the operation of a stripping column (charge reflux fractionator) without a rectifying section and reflux. A heated inert gas can be fed into the bottoms of the stripper and guided through the column in countercurrent to the entrainer rich feed, resulting in a concentrated entrainer-bottom product and a solvent (water) rich overhead product. Thin-film evaporators represent another alternative for an effective, thermally gentle and continuous recycling of a non-volatile entrainer.
188
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Fig. 5. Experimental LLE results for the system acetylated hyperbranched polyglycerol /THF /water at Tequilibrium /295.15 K [14]; , cloud points; I, concentration according to LLE measurements; hyperbranched acetylated polyglycerol: Mn /6500 g/mol, Mw/Mn / 2.1, degree of branching/0.56.
Depending on the viscosity of the solution, fallingfilm evaporators or*/for higher viscosities */rotary thin-film evaporators appear to be suitable. Although the hyperbranched polymer melts show comparatively (to other polymers) low viscosities, it might be advantageous to install a refuse worm at the rotory end of the thin-film evaporator such as those used for the stripping of epoxy resins or the degassing of polyolefins. Hyperbranched polymers can also be recycled by convection drying, using a spray dryer, a thinfilm evaporation dryer or a belt dryer. Crystallisation/precipitation, which is dependent on the underlying solid/liquid equilibrium of the polymer-solvent system, could also represent an interesting regeneration alternative. The recovery options described for a nonvolatile entrainer are competing unit operations, which have to be thoroughly assessed with regard to their investment and operation costs. A comparison with conventional separation processes for azeotropic mixtures shows, that the remarkable separation efficiencies of hyperbranched polymers
as well as the variety of energetically promising recycling options for the entrainer offer considerable potential for process and economic optimisations. Therefore, it can be concluded that hyperbranched polymers such as the polyesteramide Hybrane S1200 or the hyperbranched polyglycerol described in [14] represent promising entrainers or entrainer additives for the ethanol /water separation by means of extractive distillation. 3.2. Hyperbranched polymers as selective solvents in liquid /liquid extraction Solvent extraction is a separation process, which is in competition with distillation. However, due to its low energy consumption, solvent extraction is gaining notice. Solvent extraction is often applied if the mixture components exhibit low or high boiling points and thus distillation must be carried out under a costly low temperature or a vacuum operation. It is also applied if the mixture is a close boiling or an azeotropic system, if the components
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
189
Fig. 6. Experimental LLE, SLLE and SLE results for the ternary system hyperbranched polyester /tetrahydrofuran /water at Tequilibrium /321.15 K. LLE measurements, cloud point measurement, SLLE measurements, SLE measurements.
to be separated are thermally sensitive or if the mixture contains a key component of low concentration. Hyperbranched polymers exhibit many properties, which allow for their application as selective solvents in solvent extraction. Large selectivities, remarkable loading capacities for certain key components per mass polymer, low vapor pressures, comparatively low melt and solution viscosities, as well as thermal and chemical stability are only some of their characteristics [14]. A system of industrial importance is the azeotropic mixture of THF and water. THF is produced by dehydrocyclisation of 1,4-butanediol in the presence of an acid catalyst. The key step in purification is the breaking of the THF /water azeotrope [19]. Generally, this is achieved by twopressure (low/high pressure) distillation, extractive distillation or azeotropic distillation. In the following sections, new process approaches for the THF /water separation using hyperbranched polymers as extractive solvents are derived from LLE measurements.
3.2.1. Liquid /liquid equilibria of hyperbranched polymer /THF /water solutions Only recently [14], we discussed the potential of using hyperbranched polymers as selective solvents in the field of solvent extraction by presenting the ternary phase diagram of the system THF /water/ acetylated hyperbranched polyglycerol at 295.15 K (see Fig. 5). THF and water form a minimum boiling azeotrope containing 94.2 wt.% of THF at 0.1 MPa [19]. However, as can be seen from Fig. 5, when extracting THF from water by means of acetylated hyperbranched polyglycerol (Mn /6500 g/mol, Mw/Mn /2.1, degree of branching /0.56) the width of the miscibility gap at 295.15 K as well as the selectivity values are not large enough to break the THF /water azeotrope. Even though the solubility of acetylated hyperbranched polyglycerol in water is sufficiently small, the water solubility in the polymer and hence the water concentration in the polymer rich phase is too large for surpassing the THF /water azeotropic concentration at T/295.15 K [14].
190
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The extractive separation of THF and water by means of a hyperbranched macromolecular solvent requires a polymer, which */in contrast to the acetylated hyperbranched polyglycerol */shows a higher selectivity and a lower water absorption. Such a polymer represents the commercially available hyperbranched polyester Boltorn H3200. In Figs. 6 and 7 the ternary phase behaviour of the system THF /water /Boltorn H3200 is depicted at 321.15 and 334.15 K in the presence of a vapor phase. At both temperatures, a broad area of immisciblity originates from the binary system Boltorn H3200 /water. At 321.15 K (Fig. 6) and high polymer concentrations, the LLE turns into a solid/liquid /liquid (SLLE) and a solid/liquid (SLE) equilibrium, depending on the polymer concentration. This is due to the melting temperature of Boltorn H3200 (Tmelt,H3200 /333 K), which is larger than the system temperature of 321.15 K. Moreover, as indicated by the slope of the tielines, a solutropic effect of remarkable distinctness was found. Only a few systems such as toluene / water /acetone and n -butyl acetate/water /acetone show solutropic behaviour [20]. In the case
of the ternary system THF /water /Boltorn H3200 the solutropic effect is accompanied by an inversion of the water rich phase and the polymer rich phase. This can be explained by discussing the solubility of THF in both liquid phases and the resulting change of the phase densities in dependence on THF concentration. At low THF concentrations of the LLE region, the polymer rich phase represents the upper liquid phase and the water rich phase represents the lower liquid phase. With increasing THF concentration, for both system temperatures (Figs. 6 and 7), the THF solubility in the polymer rich phase is higher than in the polymer lean phase leading to a negative slope of tie-lines. When approaching the THF concentration in the polymer rich phase of around 50 wt.%, a change in THF distribution can be observed. Any further addition of THF results in an increasing enrichment of THF in the polymer lean phase. Therefore, the negative slope of tielines decreases until the solutropic point, where a horizontal course of the tie-lines can be observed, corresponding to equal THF concentrations in both liquid phases (see Figs. 6 and 7). The location
Fig. 7. Experimental LLE results for the ternary system hyperbranched polyester /tetrahydrofuran /water at Tequilibrium /334.15 K; I, concentration according to LLE measurements; cloud points.
Table 3 Summary of experimental LLE, SLLE, SLE and cloud point results for the systems THF /water /hyperbranched polyester Boltorn H3200 at T/321.15 K and T/334.15 K Polymer lean phase
LLE wTHF
SLLE wpolymer
LLE
SLLE
Cloud points
SLE
wwater
wTHF
wpolymer
wwater
wTHF
wpolymer
wwater
wTHF
wpolymer
wwater
wTHF
wpolymer
wwater
wTHF
wpolymer
wwater
T /321.15 K 0.2290 0.7589 0.3107 0.6757 0.3873 0.5947 0.4611 0.5173 0.5046 0.4722 0.5162 0.4584 0.5399 0.4327 0.5529 0.4212 0.5596 0.4132 0.5776 0.3920 0.6171 0.3468 0.6431 0.3199 0.6597 0.3031
0.0121 0.0136 0.0180 0.0216 0.0232 0.0255 0.0274 0.0259 0.0272 0.0304 0.0361 0.0370 0.0372
0.1274 0.1409 0.1418
0.8652 0.8500 0.8506
0.0074 0.0091 0.0076
0.1434 0.1948 0.2652 0.3218 0.3660 0.4105 0.5035 0.5803 0.6408 0.7240 0.7652 0.7939 0.8483
0.0014 0.0011 0.0031 0.0019 0.0014 0.0019 0.0049 0.0002 0.0073 0.0002 0.0065 0.0108 0.0196
0.8553 0.8041 0.7317 0.6763 0.6325 0.5876 0.4916 0.4195 0.3519 0.2758 0.2283 0.1953 0.1321
0.0747 0.0856 0.0887
0.0005 0.0008 0.0032
0.9248 0.9136 0.9081
0.7479 0.7802 0.8032 0.8200 0.8290 0.8386 0.8509
0.1883 0.1461 0.1179 0.1004 0.0879 0.0746 0.0445
0.0638 0.0737 0.0789 0.0796 0.0831 0.0868 0.1046
0.0373 0.0535 0.0541
0.0005 0.0004 0.0005
0.9622 0.9461 0.9454
T /334.15 K 0.1008 0.8940 0.2665 0.7204 0.3019 0.6850 0.3740 0.6100 0.4740 0.5013 0.5298 0.4382 0.5893 0.3801 0.6237 0.3425 0.6470 0.3195
0.0052 0.0131 0.0131 0.0159 0.0247 0.0320 0.0306 0.0338 0.0335
0.0763 0.1319 0.1546 0.1901 0.2847 0.4022 0.5637 0.7061 0.8088
0.0010 0.0012 0.0013 0.0005 0.0024 0.0010 0.0040 0.0119 0.0514
0.9226 0.8669 0.8441 0.8093 0.7129 0.5968 0.4323 0.2820 0.1398
0.7030 0.7219 0.7618 0.7781 0.7930 0.7992 0.8090 0.8276 0.8305
0.2401 0.2144 0.1646 0.1414 0.1214 0.1121 0.1018 0.0732 0.0664
0.0569 0.0637 0.0736 0.0805 0.0856 0.0887 0.0892 0.0992 0.1031
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
Polymer rich phase
191
192
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Fig. 8. Distribution of THF between the polymer rich and the polymer lean phase for ternary liquid /liquid equilibria of different hyperbranched polymers in THF /water.
of the solutropic point shows a slight temperature dependence. For the equilibrium temperature of 321.15 K (Fig. 6) the solutropic point is located at 54 wt.% THF, whereas for the system temperature of 334.15 K (Fig. 7) the solutropic THF concentration amounts to 60 wt.% (see also Fig. 8). The increased enrichment of THF in the lower polymer lean phase also results in a decreasing density of this phase. Hence, the densities of both liquid phases approach each other leading to similar phase densities in the solutropic area. For THF concentrations above the solutropic point, the THF solubility in the water rich phase is still higher than in the polymer rich phase (positive slope of tie-lines). Therefore, at around 65 wt.% of THF, the density of the water rich phase falls below the density of the polymer rich phase and phase inversion is observed. Apart from these thermodynamic phenomena, it is noteworthy concerning the THF /water separation, that for both system temperatures all phase compositions of the polymer rich phase indicate
the breaking of the atmospheric THF /water azeotrope (see experimental results in Table 3 but on polymer-free basis). For the equilibrium temperatures of 321.15 and 334.15 K, the distribution of THF between the polymer rich and the polymer lean phase is depicted in Fig. 8. Below 50 wt.% of THF, for both polymer solutions, the THF solubility in the polymer rich phase is higher than in the water rich phase. As can be seen for the Boltorn H3200 system, the THF solubility in the polymer rich phase increases with the temperature. At the solutropic point the diagonal is intersected, indicating equal THF concentrations in both liquid phases7. Above around 60 wt.% of THF, due to a higher THF solubility in the polyester lean phase,
7
In the case of the hyperbranched polyglycerol solution, no solutropic effect was measured. Therefore, the intersection with the diagonal represents the critical point of the mixture which can be estimated from Fig. 5.
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
193
Fig. 9. Selectivity vs. weight fraction of THF for liquid /liquid equilibria of different hyperbranched polymers in THF /water.
the THF concentration of the water rich phase is larger than for the polymer rich phase. Eventually, the critical point of the Boltorn H3200 mixture, at which the THF concentration in both liquid phases are equal, appears. The preference of Boltorn H3200 for THF as the preferred interaction partner can be recognised from the activity coefficients at infinite dilution. Activity coefficients at infinite dilution of a solute i, g i , provide information about the intermolecular energy between the solvent and the solute and are used, in particular, for the selection of solvents for extraction and extractive distillation [21]. For two different temperatures, T /345.15 K and T /357.17 K, activity coefficients of THF and water infinitely diluted in the hyperbranched polymer Boltorn H3200 were determined [15]. While g water increases from 0.17 (for T /345.15 K) to 0.21 (for T/357.17 K), g THF does not show
a significant temperature dependence (g THF /0.03 for 345.15 K B/T B/357.17 K). The effectiveness of THF extraction by hyperbranched polymer can be characterised by its selectivity S , which is a measure of the ability of the polymer to separate THF from water. For solvent extraction, the selectivity, as defined in Eq. (3), is of comparable importance to the separation factor used in distillation. STHF;water
KTHF w Kwater w
L1 THF L2 THF
wL2 water wL1 water
L1:polymer rich phase; L2:polymer lean phase (3) Based on Table 3, in Fig. 9 the selectivity STHF,water is plotted versus the weight fraction of THF in the polymer rich phase. Unlike hyperbranched acetylated polyglycerol, the Boltorn
194
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Fig. 10. Separation of THF /water by means of a hyperbranched polymer as extraction solvent; A1, A2: countercurrent distillation column, B: mixer /settler, C: polymer /solvent separation unit (e.g. evaporator, dryer); Compositions: THF/polymer/water, concentrations in wt.%.
H3200 system shows remarkable selectivities for the THF /water separation, which gradually decreases with increasing THF concentration in the polymer rich phase. Therefore, it can be concluded that the hyperbranched esterified polyester Boltorn H3200 meets the requirements of an efficient extraction solvent. Thus, new process approaches for the separation of THF /water by means of a hyperbranched polymer can be derived (see next section). 3.2.2. New process approaches for the THF /water separation Based on Fig. 6, the following three separation processes could be suggested for separating a 50 wt.% THF /50 wt.% water mixture: Fig. 10 shows a separation process on the basis of azeotropic distillation. The first distillation column separates the THF /water feed into water as the bottom product and the azeotrope-close THF /water mixture (wTHF :/0.94) as the overhead product. According to Fig. 10, the latter
stream is fed into the second atmospheric countercurrent distillation column, which is operated with a THF-rich reflux. The reflux represents the upper polymer-rich phase in a mixer-settler, whose constant phase compositions could be guaranteed by adjusting the liquid mixture at 321.15 K to that specific tie-line, which leads to an upper liquid phase characterised by wTHF /0.540, wpolymer / 0.433, wwater /0.027 and a lower liquid phase characterised by wTHF /0.503, wpolymer /0.005, wwater /0.492 (see also Fig. 6 and Table 3). Due to the THF-rich reflux, the overall THF concentration in the second distillation column is larger than the azeotropic THF concentration (wTHF /0.942). Therefore, (in spite of the presence of a second liquid phase and a resulting additional mass transfer resistance) a water-free THF /Boltorn H3200 mixture can be received as the bottom product and the minimum boiling azeotrope as the overhead product. The bottom product is separated e.g. by means of a thin-film evaporator into the pure THF product and a highly concentrated
M. Seiler et al./Separation and Purification Technology 30 (2003) 179 /197
195
Fig. 11. Separation of THF /water by means of a hyperbranched polymer as extraction solvent, A1, A2: countercurrent distillation column, B: mixer /settler, C: polymer /solvent separation unit (e.g. evaporator, dryer); Compositions: THF/polymer/water, concentrations in wt.%.
polymer solution8, which is conveyed at T / 333K9 into the mixer-settler. In order to save the reboiler of the column A1, a partial flow of the THF vapor product could be fed into the bottom of A1. The liquid mixture, consisting of the regenerated polymer and the condensed overhead product of the second distillation column, should meet the phase composition criteria described above, so that the lower polymer free, liquid phase of the mixer-settler shows the same composition as the initial THF /water feed solution. Therefore, both streams, the lower liquid phase of the mixersettler and the THF /water feed, can be mixed in accordance to the overall mass balance and sub8
Since the mixture in the mixer /settler must have a constant composition, the overall mass balance determines the amount of THF in the highly concentrated polymer solution. Generally, a concentrated polymer solution containing about 10 wt.% of THF appears desirable, since the viscosity of the concentrated polymer stream as well as the energy input of the evaporator have to be taken into account. Future work will focus on these aspects of process optimization (see Section 4). 9 Boltorn H3200 is thermally stable up to 520 K. The melting point of this semi-crystalline polymer is 333 K.
sequently fed into the first distillation column (see Fig. 10). The second separation scheme, depicted in Fig. 11, represents a process with the same unit operations as in Fig. 10. But since the unit operations in Fig. 11 are arranged in a different order and the solvent extraction is based on another tie-line, the second separation scheme (Fig. 11) might be energetically superior to conventional extraction processes and to the separation scheme of Fig. 10. As can be seen in Fig. 11, the first separation step represents the liquid-liquid demixing of the THF /water feed, which is mixed with the regenerated polymer, in a mixer-settler at 321.15 K. In this process another tie-line from Fig. 6 is chosen for the single step extraction, leading to an upper liquid phase characterised by wTHF / 0.461, wpolymer /0.517, wwater /0.022 and a lower liquid phase characterised by wTHF /0.322, wpolymer /0.002, wwater /0.676. The latter waterrich phase is separated by means of a small atmospheric countercurrent distillation column, which */within two or three theoretical stages */ leads to water as the bottom product and a THF /
196
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water mixture, similar in composition to the feed solution, as the overhead product. The upper polymer-rich phase of the mixer settler with wTHF /0.955 /wTHF,azeotrope /0.942 is fed into a second distillation column. The bottom product of this column, a THF /polymer mixture, is separated (for instance by means of an evaporator or dryer). The yielded polymer is added to the columns’ overhead product (i.e. the low boiling azeotropic THF /water mixture), so that the underlying tieline of the single step extraction is met and the overall mass-balance is satisfied. When necessary, additional polymer is added in order to reach the selected tie-line. The THF /polymer separation is realised in accordance with Fig. 10. A third separation process is conceivable, which*/apart from the separation of the upper liquid phase of the solvent extraction in Fig. 11*/ is equivalent to the latter separation scheme. In this third approach, the upper polymer rich phase from the liquid/liquid extraction is separated by means of (shear) crystallisation into a liquid THF product and a solid phase, containing all water and most of the polymer. The solid mixture is fed into an extruder. Here, devolatilisation allows the water /polymer separation, so that the polymer can be recycled. Further research on the underlying solid-liquid equilibria will investigate the feasibility of crystallisation for the THF /water separation.
4. Conclusions and future work Hyperbranched polymers represent a promising class of highly selective macromolecules, which can be used for the optimisation of a variety of separation processes. Properties such as solubility, capacity, selectivity, melt and solution viscosity, thermal stability as well as glass transition temperature can be tailored according to the individual application. In this work the potential of commercially available hyperbranched polymers in the field of extractive distillation and solvent extraction is demonstrated. In contrast to Hybrane H1500 and Boltorn H20, the hyperbranched polymers Hybrane S1200 and hyperbranched polyglycerol exhibit remarkable
separation efficiencies for the ethanol /water system. Both hyperbranched representatives, Hybrane S1200 and hyperbranched polyglycerol, easily break the azeotropic phase behaviour of the ethanol /water system due to selective interactions with water. For xethanol /0.3, a considerable increase of the separation factor aethanol,water was observed when adding Hybrane S1200 or hyperbranched polyglycerol to the ethanol/water solution. In case of Hybrane S1200 this increase proved to be of the same magnitude as that of the conventional entrainer 1,2-ethanediol, which confirms the suitability of using Hybrane S1200 as an entrainer or entrainer additive for the separation of azeotropic mixtures by extractive distillation. Due to the use of a non-volatile entrainer, the entrainer regeneration can be carried out by stripping, evaporation, drying or crystallisation. Moreover, the main column does not require a section for the separation of the low boiling component from the entrainer, since a polymeric entrainer cannot pollute the columns’ overhead product. Therefore, extractive distillation using one of the suggested hyperbranched polymers as entrainer indicate a potential for cost-savings in comparison to conventional extractive distillation processes. The separation of the azeotropic system THF / water by means of solvent extraction is also studied. The hyperbranched aliphatic polyester Boltorn H3200 is used as a solvent. Boltorn H3200 exhibits a remarkable selectivity and capacity, which allows for the breaking of the THFwater azeotrope by single stage extraction. The purification of the extract and the raffinate can be achieved by combining the extraction unit either with distillation or crystallisation steps. For the system THF/water /hyperbranched polyester, a remarkably distinct solutropic phenomenon is observed. Future work will focus on evaluating the suggested processes for separating azeotropic mixtures by means of hyperbranched polymers. ASPEN PLUS† simulations will be carried out to energetically optimise the individual separation approach and to discuss their advantages and
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disadvantages in contrast to conventional separation processes. [8]
Acknowledgements The authors would like to thank Perstorp Speciality Chemicals AB for providing hyperbranched aliphatic polyesters. Thanks especially to B. Pettersson and H.C. Bjoernberg, Perstorp, for useful discussions. We also appreciate the support of Dr P.E. Froehling, DSM, who kindly provided hyperbranched polyesteramides. Experimental HSGC-assistance of Dipl.-Ing. A. Kavarnou is gratefully acknowledged.
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